From the point of view of safety, in the case of automatic transmissions there are known devices which prevent a shifting into reverse gear through the setting of the selecting slide or manual valve in the position R by the driver when there is an excessively high driving speed and the vehicle rolls fowardly too quickly. Such devices are necessary in spite of the increased structure involved, in order to avoid damage in the transmission line, primarily in the gears, in the case of impermissible shifting into reverse gear.
In a known automatic transmission 4HP22 with an electronic-hydraulic control--ATZ 85 (1983) pages 401 to 405--such an inadmissible shifting is prevented by providing an additional solenoid valve and an additional reverse-speed safety valve especially designed for this purpose.
This arangement has proven to be suitable, since shifting of the manual valve into reverse does not lead to shifting in the transmission as long as the vehicle continues to rolls forwardly at a speed of over 8 km/h. However, the device is relatively complex.